1. Field of the Invention
The present invention relates to the fabrication of semiconductor devices. More particularly, the present invention relates to a method of and apparatus for determining optimum exposure conditions in a photolithographic process.
2. Description of the Related Art
The manufacturing of electronic devices such as semiconductor devices or liquid crystal displays typically includes a photolithography process. Photolithography generally involves exposing a photosensitive film, disposed on a semiconductor or glass substrate, to light directed through a photo mask or a reticle (hereinafter, referred to as a ‘reticle’) to transfer an image of the reticle to the film. The photosensitive film is then developed to produce a pattern for use as a mask during the subsequent processing of the substrate. In such a photolithography process, the exposure light must have energy capable of chemically changing the photosensitive film such that an exact focused image, i.e., an image having the desired pattern, will be transferred to the photosensitive film.
In a conventional technique to determine a correlation between the focus and the light energy, at least one condition of a mock photolithography process is continuously changed so that different regions of a photosensitive film (shots) are sequentially exposed under varying conditions. For instance, a condition related to the focusing of the image and/or the exposure amount (time during which a shutter is held open) is changed for each shot, thereby forming corresponding pattern images at the different shots. Then, respective mask patterns are produced by developing the photosensitive film. The dimensions of the mask patterns thus produced are measured using an optical microscope or a scanning electron microscope (SEM) to ascertain the best mask pattern. The optimum exposure condition is determined from the best mask pattern as a correlation between the focus and the exposure amount.
However, measuring the developed mask patterns using a SEM requires a great amount of time. Furthermore, the work is repetitive, must be constantly checked for accuracy, and corrections must be made throughout the course of the measuring processes. Still further, an actual pattern for use as a mask on a substrate is more complicated, in terms of the various shapes and line widths thereof, than the pattern used in the mock photolithographic process. Thus, the conventional measurement technique also must rely on statistical analysis to determine the optimum exposure conditions for the actual photolithography process.